Heretofore, large NaI:Tl crystals have been used in scintillation counters to convert deep well gamma radiation into photons which were amplified by peripheral photo-multiplier tubes. The gamma-to-charge conversion is a complex, multi-step process involving:
1. an encounter with an iodide atom, PA1 2. release of an outer shell electron, PA1 3. excitation of NaI by the free electron, PA1 4. migration of the excitation energy to a Tl dopant center, PA1 5. emission of a photon within the spectral range of a photo-multiplier phosphor, and PA1 6. cascade multiplication within the stages of a photo-multiplier tube.
While these sophisticated counters are highly useful in research; their size, complexity, cost, and short life time have limited their commercial applications.
The extended dimensions required to combine the NaI crystal with a discrete photo-multiplier tube, reduces realizable cell density. In addition, the NaI crystals required a hermetic perpheral seal. Water traces cause the crystal to lose its scintillation properties. Small infiltration rates can result in water accumulation within the crystal, which degrades the crystal structure. The absolute "dry room" conditions essential for the proper crystal growing, machining, and packaging, contribute to the already high cost of the scintillation units.
Single wire Geiger counter chambers offer a direct conversion of gamma radiations to electrons. However, the high collection voltage between the center wire anode and the outer cylinder cathode causes complete, selfsustaining electrical breakdown in response to any radiation above the detection threshold. Th vigorous avalanche proximate the centr anode spreads spontaneously along the wire, and must be extinguished after each count by temporarily reducing the collection voltage. This de-ionization relaxation period after each detection is "dead time" and severely limits the upper count rate of Geiger counters.
For the conventional wide angle applications, the peripheral cylindrical housing forms the radiation window. The window must be thin to permit penetration by gamma radiation; and therefore cannot withstand the internal expansion force of a pressurized conversion gas. Most Geiger counters are limited to near atmospheric internal pressures and hence have low gamma conversion ratios.
The total ionization associated with each detection cycle generates molecular degradation within the conversion gas; which in combination with the tremendous acceleration proximate the center anode wire, causes slow structural degradation of the anode surface. Geiger counters typically have shorter useful lifetimes than proportional counters which operate at lower voltages with less ionization.